On the left, go to the library. Choose "new media." Select browse to find the file you want. Upload. Image will show up in the library. Find the image in the library. Select "edit." Down at the bottom you'll find the URL for that image. Copy that. create a comment. Post comment. Go back and edit your comment. Use the "img" button to add image. Update and close.

You can add that image directly into a new post without editing.

Edit:
Try to keep the image small. Less than 1000 pixels wide. Otherwise, it's just too big for the screen. There are image editing tools in the library as well.